1. Field of the Invention
The present invention relates to an antenna and, more particularly, to an NRD guide (nonradiative dielectric waveguide) antenna in the millimeter-wave band used for collision prevention apparatus of automobiles, and the like.
2. Description of the Related Art
Antennas used in a conventional millimeter-wave band NRD guide and shown in FIGS. 8 and 9. Shown in FIG. 8 are planar conductors 71 and 72. A dielectric strip 73 is interposed between the conductors 71 and 72, forming an NRD guide. Further, one end of the NRD guide is connected to a transmit-receive circuit (not shown), and the dielectric strip 73 projects from between the conductors 71 and 72, thus forming an antenna 74. The antenna 74 constructed in this way radiates electromagnetic waves along the direction of the length of the dielectric strip 73.
Shown in FIG. 9 is a dielectric strip 81. Planar conductors 83 and 84 are mounted on the top surface and under surface of the dielectric strip 81, respectively, thus forming an NRD guide. Notches 81a through which electromagnetic waves radiate are formed along the horizontal direction (along the X-axis direction in FIG. 9) of the dielectric strip 81. A coaxial line 82 for supplying power is mounted on the side of one end portion of the dielectric strip 81. Further, a plurality of openings 85 are formed in the conductor 83 on the top surface in such a manner as to be parallel to the direction of the length (along the Y-axis direction in FIG. 9) of the dielectric strip 81, thus forming a plane antenna 86.
From the plane antenna 86 constructed as described above, electromagnetic waves are radiated from the dielectric strip 81 in the horizontal direction (in the X-axis direction in FIG. 9), and when openings 85 are formed in the conductor 83 at intervals of one wavelength thereof, electromagnetic waves are radiated via the openings 85 in the vertical direction (in the Z-axis direction in FIG. 9) of the plane antenna 86.
However, in the above-described conventional antenna 74, the radiation direction of electromagnetic waves is only in the direction of the length of the dielectric strip 73. Therefore, if the antenna 74 is used in a collision prevention apparatus of an automobile and mounted in such a way that the radiation direction of electromagnetic waves is directed in the moving direction of the automobile, the antenna 74 is disposed parallel to the moving direction of the automobile, and the sides of the conductors 71 and 72 of the antenna 74 project, for example, into the engine compartment. Thus, the mounting position of the antenna 74 is limited.
Further, since the plane antenna 86 is used in the millimeter-wave band, machining of the notches 81a formed in the dielectric strip 81 and the openings 85 of the conductor 83 requires high precision. Also, since a plurality of openings 85 are required, the surface areas of the conductors 83 and 84 become wide, and the conductors 83 and 84 flex due to insufficient strength of the conductors 83 and 84, causing variations in the antenna characteristics.
Furthermore, the gain of the antenna 74 depends upon the length of the dielectric strip 73. When the amount of loss in the dielectric strip 73 is taken into consideration, only 20% to 50% can be obtained as the antenna efficiency. If it is desired to increase gain in the plane antenna 86, its shape becomes enlarged.